Process and apparatus for making alkali cellulose in sheet form

ABSTRACT

Improved process and apparatus for making alkali cellulose comprises continuously steeping indefinite length sheets of cellulose in an alkali hydroxide bath, generally 17-25% sodium hydroxide, and then passing the cellulose strip through a chamber where it is subjected to extremely rapid mercerizing conditions, including an RF energy field, to produce depolymerization thereof. Upon emergence from the steep bath, the cellulose passes to an excess solution removing means, such as one or a series of pairs of nip rolls where the excess solution is expressed from the cellulose and the amount of absorbed alkali is controlled. Preferably, the excess solution is recovered and returned to the steeping baths. A second, lower concentration, steep bath may follow the rapid mercerization step.

This is a continuation of application Ser. No. 576,200, filed May 9,1975, now U.S. Pat. No. 4,163,840 issued Aug. 7, 1979.

This invention pertains to improved process and apparatus for producingalkali cellulose as a first manufacturing operation in the production ofregenerated cellulose products, such as viscose rayon. This inventionpertains more specifically to process and apparatus involving alkalisteeping of sheet cellulose in a continuous manner followed bycontinuous mercerization in sheet form.

A variety of processes and apparatus have been proposed heretofore forsteeping cellulose in an alkali hydroxide solution to produce alkalicellulose which is subsequently xanthated, filtered and formed, moreparticularly spun, into a product, such as continuous or staple yarn.One common procedure is to vertically stand a plurality of sheets ofsuch cellulose in a batch-wise manner, and immerse the sheets in analkali hydroxide solution bath. Continuous steeping has also beenproposed, either by immersion of shredded cellulose in alkali hydroxidesolution and later expressing the excess solution from the steepedslurry product or by continuously applying or contacting or immersingsheet cellulose of indefinite length in a steeping spray or bath. Suchprocesses or parts thereof are seen, for example, in U.S. Pat. Nos.2,614,102--Schlosser et al, 2,499,010--Weeldenburg, 2,143,863--Collingset al and 2,149,178--Morton.

Following steeping it has been customary to age (mercerize) the alkalicellulose to bring about a depolymerization of the cellulose from a DPof 700-1000 to approximately 300-450, depending on the product to bemade. Normally, the alkali cellulose is shredded and either collected inbatches in cans, or treated continuously on belts or in towers forperiods of time depending on the temperature and the catalysts present.Such periods of time have ranged from 20 to 80 hours.

It is known that catalysts such as iron, manganese, cobalt, salts, etcetera, catalyse the oxidation reaction and markedly reduce aging time.Similarly, higher temperatures and higher caustic content of alkalicellulose reduce aging time.

It has also been known that after mercerizing, the caustic content canbe reduced by resteeping in a caustic solution of approximately 10-12%*sodium hydroxide, without adversely affecting its ability to formcellulose xanthate, and resulting in fewer by-products in the form ofsulfur compounds being formed by the reaction of free excess sodiumhydroxide with carbon disulfide. A process of this type is disclosed bySihtola in U.S. Pat. Nos. 3,600,379 and 3,728,330.

It is the object of the present invention to provide an improvedsteeping and mercerizing process and apparatus for making alkalicellulose crumbs in a continuous, effective and efficient manner.

Previously disclosed processes for steeping and mercerizing continuouslyin sheet form always involved either low rates of through-put or verycumbersome and space-requiring means of mercerizing the continuous sheetprior to xanthation or to a second steeping followed by xanthation.

The process of the present invention provides for very rapid mercerizingso that continuous sheet steeping and mercerizing is a practical andeconomic operation.

It is therefore the general object of the present invention to providean improved steeping-mercerization process and apparatus for makingalkali cellulose in a continuous, effective and efficient manner.

More specifically, it is an object of this invention to provide aneffective and efficient steeping method for uniformly convertingindefinite lengths of sheet cellulose to the alkali cellulose form ofdesired degree of polymerization, ready for xanthation and spinning intoa staple or continuous filamentary product.

These objects, and others which will be apparent in the course of thesubsequent description of the invention, are met, briefly, by a processwherein sheet cellulose of indefinite length, and any desired width ispassed first through a first stage steep at a rate of about 1 foot persecond. The steep bath, which is on the order of 10-15 feet in length,consists of 17-25% alkali hydroxide solution. The caustic soda solutioncontains manganese or cobalt catalysts, or other chemical additives toaccelerate mercerizing, and is in the normal steeping temperature rangeof 18°-50° C., preferably about 25° C. After steeping, excess solutionis removed from the sheet by a series of pairs of nip rolls, each ofwhich subjects the sheet to a successively greater pressure.

The pressures used are such as to give 3.2 to 2.6 press ratios. Pressratio is the weight ratio of pulp after pressing divided by the weightbefore steeping. Preferably also, a vacuum suction means is disposedjust upstream of the nip roll pairs for removing excess solutionexpressed in the nip rolls.

The sheet then moves to the mercerizing chamber in which it is passedbetween electrodes subjecting it to electromagnetic (RF or microwave)energy at a frequency of 10⁷ -10¹¹ hertz. This raises the temperature tobring about the desired degree of polymerization in a very short time,more specifically, about 1/2-4 minutes, and preferably about 2 minutes.

Following the mercerization, the alkali cellulose sheet can then proceeddirectly to the xanthation step, or it can proceed through a secondsteeping operation, similar in apparatus to the first steepingoperation, in which it is treated with a catalyst-free, more dilutecaustic solution (on the order of 8-15% sodium hydroxide) displacing apart of the excess free caustic soda, pressed to a press ratio of 3.0 to2.4. It then is transferred to the xanthation stage.

Excess solution removed from the alkali cellulose product isrecirculated to the steeping baths with the usual means of control ofhemi-cellulose. These means for handling the sheet cellulose and passingit through the successive treatment steps may also include means forhandling a plurality of indefinite length sheets of cellulose superposedover one another through the process. Means should also be included inthe case of such superposed plurality of sheets to maintain such sheetsin a spaced apart relation as they are immersed in and passed througheach of the alkali hydroxide baths.

For a better understanding of the invention, reference should be made tothe following detailed description thereof taken in conjunction with theappended claims, and the FIGURE, which is a diagrammatic illustration ofthe process and apparatus of the preferred form of the presentinvention.

Referring more specifically to the FIGURE, there is shown alternatecellulose pulp sheet supply rolls 1, 1a, with drive rolls 1b, 1c,supplying strips of pulp of indefinite length and any desired width,respectively over idler rolls 2, 2a and through feed roll pairs 3, 3a toa first alkali hydroxide steeping bath S₁, supported therein by endlessguide belts 4 and 5, comprised of an open mesh glass or polypropylenefabric. Drive rolls 1b, 1c, feed roll pairs 3, 3a, and endless guidebelts 4 and 5 are synchronized to convey the pulp sheet at a relativelyconstant velocity.

Preferably first stage steep bath S₁ is an 18-20% sodium hydroxidesolution and the length of immersed sheet and its speed of travel isselected to provide an immersion time of 8-15 seconds. The nominalvelocity of the pulp sheet is about 1 ft/sec and the length of travel inbath S₁ is about 10-15 feet. Leaving steep bath S₁, the sheet issupported by another open mesh belt 6 as it passes through a series ofpairs of nip rolls 7, 8, and 9, the rolls of each pair being spaced andurged toward one another to exert succeedingly higher pressure on thesheet product to express solution therefrom without physically damagingthe sheet. Generally, a progressively lower press ratio, in the range of3.2 to 2.6, is used. One or more catch pans may be provided to recoverdrippings of excess alkali hydroxide from nip rolls 7, 8, and 9.Preferably, however, vacuum suction means V are provided upstream of oneor more of each pair of nip rolls to remove excess alkali hydroxideexpressed from the alkali cellulose strip passing therethrough. Theexcess alkali hydroxide solution recovered in this manner may berecirculated to the make-up stream for the steeping bath in aconventional manner.

The alkali cellulose sheet is then picked up by porous endless glassbelt support 10 for mercerization. Mercerization may be promoted byintroducing into the steep liquor of bath S₁ depolymerizationaccelerators, such as manganese and cobalt catalysts. An elevatedtemperature, on the order of 45° C., for example, may also be maintainedto promote depolymerization in accordance with known methods.Mercerization or depolymerization is achieved rapidly by impartingenergy to the cellulose in the form of electromagnetic radiation, or RFenergy, from electrodes RFE, in a frequency range of 10⁷ -10¹¹ hertz,and sufficient power to bring about the desired degree of polymerizationduring the time allowed in the mercerization stage. Other known methodsof accelerating mercerization may be used in conjunction with RF energy.

Following RF depolymerization, some cooling stage may be required priorto xanthation. In the preferred form of this invention, this cooling iseffected by the second steeping bath, which is maintained at atemperature selected to maintain optimum xanthation conditions(generally between 30° and 40° C.).

The partially depolymerized alkali cellulose sheet is then forwarded toa second steeping bath of alkali hydroxide solution (in the preferredform of the present invention). Bath S₂ comprises about 9-11% sodiumhydroxide. Support belts 11 and 12 comprised of an open mesh woven glassor polypropylene fabric, are provided to forward the sheet materialthrough the second steeping bath. The steeping time and excess solutionremoving means (13, 14, 15 and 16) in steeping bath S₂ are approximatelythe same as those in bath S₁. However, the final press ratio in niprolls 16 may be as low as 2.4 to minimize excess caustic in thesubsequent xanthation step.

Similar vacuum suction removal means V, as those described with respectto bath S₁ may be provided to recover excess alkali hydroxide and torecirculate or recover excess solution expressed from the alkali sheetby nip roll pairs 14, 15 and 16 and the vacuum pick-up.

The depolymerization process described herein, using RF energy, ispreferably used in conjunction with a travelling sheet steeping processas described herein. However, this RF depolymerization may also be usedin conjunction with other processes such as those in which the alkalicellulose is in crumb form.

While this invention has been described with respect to selectedembodiments and examples, it should be understood that various minormodifications and adaptations of the process and apparatus of thepresent invention may be made without departing from the true spirit andscope thereof. For example, while this invention has been illustrated inan embodiment involving a single cellulose strip continuously fedthrough and treated in accordance with the apparatus and process of thisinvention, a plurality of such strips, superposed and spaced apart fromone another, may also be utilized. Accordingly, the appended claims areintended to be construed to cover all such variations and adaptations ofthe invention which may be made by those skilled in the art withoutdeparting from the true spirit and scope thereof.

We claim:
 1. An apparatus for mercerizing alkali cellulose comprising:amercerizing chamber including means for generation of an RF energy fieldtherein, and having means for subjecting said cellulose to mercerizingconditions including said RF energy field.
 2. In an apparatus formercerizing alkali cellulose, the improvement comprising a means forenhancing said mercerization process, said means adapted to impart RFenergy to said alkali cellulose.
 3. Apparatus for converting sheets ofcellulose of indefinite length to alkali cellulose consisting of(a) afirst bath of 17-25% alkali hydroxide solution, said bath being adaptedto permit said sheet to be passed therethrough and immersed therein, (b)means for expressing excess solution from said cellulose sheet followingimmersion of said sheet in said first bath, (c) a mercerizing chamberand means therein for subjecting cellulose passing therethrough tomercerizing conditions including an RF energy field, (d) means forcontinuously supplying said cellulose sheet and forwarding it throughsaid bath, while immersing it therein, through said mercerizing chamberand to said excess solution expressing means.
 4. Apparatus, as recitedin claim 3, wherein said excess solution expressing means consists of aseries of pairs of nip rolls, the pressure between the rolls of the lastof said pairs being higher than that in each of the preceding pairs ofrolls and the pressure between the rolls in each of the preceding rollsbeing less than that of the next succeeding pair in said series, saidpressures being selected to provide progressively lower press ratios,the final press ratio within the range 3.2 to 2.6.
 5. Apparatus, asrecited in claim 3, further including a second bath, said second bathconsisting of 8-15% alkali hydroxide solution and being adapted topermit said sheet to be passed therethrough, following mercerization,and immersed therein.
 6. Apparatus, as recited in claim 3, wherein saidsupply and forwarding means further include means for supplying andforwarding a plurality of superposed cellulose sheets, and formaintaining said sheets spaced apart from one another as they are passedthrough said alkali hydroxide baths.